Abstract: A cobalt based Fischer-Tropsch catalyst which including a porous catalyst support and metallic cobalt crystallites within the support. The catalyst has a proportion of its cobalt in reducible form, with this proportion being expressible as &OHgr; mass %, based on the total pre-reduction catalyst mass. The catalyst also has, when freshly reduced, a mono-modal Gaussian metallic cobalt crystallite size distribution, a metallic cobalt surface area, in m2 per gram of catalyst, of from 0.14 &OHgr; to 1.03 &OHgr;, and a catalyst geometry that ensures a stabilized Fischer-Tropsch synthesis effectiveness factor of 0.9 or greater.